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We describe the objectives, design and predicted performance of Clover, which is a ground-based experiment to measure the faint ``B-mode'' polarisation pattern in the cosmic microwave background (CMB). To achieve this goal, clover will make polarimetric observations of approximately 1000 deg^2 of the sky in spectral bands centred on 97, 150 and 225 GHz. The observations will be made with a two-mirror compact range antenna fed by profiled corrugated horns. The telescope beam sizes for each band are 7.5, 5.5 and 5.5 arcmin, respectively. The polarisation of the sky will be measured with a rotating half-wave plate and stationary analyser, which will be an orthomode transducer. The sky coverage combined with the angular resolution will allow us to measure the angular power spectra between 20 < l < 1000. Each frequency band will employ 192 single polarisation, photon noise limited TES bolometers cooled to 100 mK. The background-limited sensitivity of these detector arrays will allow us to constrain the tensor-to-scalar ratio to 0.026 at 3sigma, assuming any polarised foreground signals can be subtracted with minimal degradation to the 150 GHz sensitivity. Systematic errors will be mitigated by modulating the polarisation of the sky signals with the rotating half-wave plate, fast azimuth scans and periodic telescope rotations about its boresight. The three spectral bands will be divided into two separate but nearly identical instruments - one for 97 GHz and another for 150 and 225 GHz. The two instruments will be sited on identical three-axis mounts in the Atacama Desert in Chile near Pampa la Bola. Observations are expected to begin in late 2009.

We describe the objectives, design and predicted performance of Clover, a fully-funded, UK-led experiment to measure the B-mode polarisation of the Cosmic Microwave Background (CMB). Three individual telescopes will operate at 97, 150 and 225 GHz, each populated by up to 256 horns. The detectors, TES bolometers, are limited by unavoidable photon noise, and coupled to an optical design which gives very low systematic errors, particularly in cross-polarisation. The telescopes will sit on three-axis mounts on a site in the Atacama Desert. The angular resolution of around 8 ́ and sky coverage of around 1000 deg2 provide multipole coverage of 20

CLOVER aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. CLOVER consists of two polarimeters. One operates at 97 GHz, using fintine-coupled Transition Edge Sensors (TES). The other has a combined 150/220-GHz focal plane populated by radial-probe coupled TES detectors. The 97-GHz instrument will have 100 feedhorns and 200 detectors while the combined 150 and 220-GHz instrument will have a total of 200 horns. To achieve the target NEP of 1.5 x 10(-17) W Hz(-1) the 97-GHz detectors will have a transition temperature of 190 mK and will operate with a base temperature of similar to 100 mK. CLOVER's detectors are fabricated on 225-micron silicon substrates. In the 97-GHz instrument a finline transition feeds a microstrip which is terminated by a matched resistor on the silicon nitride island that carries the TES. Each detector is fabricated as a single chip to ensure a 100% operational focal plane. The detectors are mounted in linear modules made of copper which form split-block waveguides. Each detector module contains a time-division SQUID multiplexer to read out the detectors. The multiplexed signals are further amplified by SQUID series arrays. The first prototype detectors for CLOVER have a transition temperature of 350 mK and were fabricated to validate the detector design and the polarimeter technology. We have characterised these detectors in a dedicated test facility. The CLOVER testhed contains cryogenics similar to those in the final instrument: a pulse-tube cooler, He-7 sorption fridge, and a mini dilution fridge so that the detectors are tested in a realistic environment. The test bed has a cryogenic blackbody source with band-defining filters for optical testing. As well as the multi-channel electronics that will be used on the final instrument the test bed has an analogue SQUID readout which allows us to characterise the readout fully. We discuss the results of the detector tests and the design changes needed to achieve the required sensitivity.

CLOVER aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. CLOVER consists of two polarimeters. One operates at 97 GHz, using finline-coupled Transition Edge Sensors (TES). The other has a combined 150/220-GHz focal plane populated by radial-probe coupled TES detectors. The 97-GHz instrument will have 100 feedhorns and 200 detectors while the combined 150 and 220-GHz instrument will have a total of 200 horns. To achieve the target NEP of 1.5 x 10 -17 W Hz-1 the 97-GHz detectors will have a transition temperature of 190 mK and will operate with a base temperature of ∼ 100 mK. CLOVER's detectors are fabricated on 225-micron silicon substrates. In the 97-GHz instrument a finline transition feeds a microstrip which is terminated by a matched resistor on the silicon nitride island that carries the TES. Each detector is fabricated as a single chip to ensure a 100% operational focal plane. The detectors are mounted in linear modules made of copper which form split-block waveguides. Each detector module contains a time-division SQUID multiplexer to read out the detectors. The multiplexed signals are further amplified by SQUID series arrays. The first prototype detectors for CLOVER have a transition temperature of 350 mK and were fabricated to validate the detector design and the polarimeter technology. We have characterised these detectors in a dedicated test facility. The CLOVER testbed contains cryogenics similar to those in the final instrument: a pulse-tube cooler, He-7 sorption fridge, and a mini dilution fridge so that the detectors are tested in a realistic environment. The test bed has a cryogenic black-body source with band-defining filters for optical testing. As well as the multi-channel electronics that will be used on the final instrument the test bed has an analogue SQUID readout which allows us to characterise the readout fully. We discuss the results of the detector tests and the design changes needed to achieve the required sensitivity.